1. Technical Field
The present invention relates to a system and a method for treating dust contained in extracted cement kiln combustion gas, and more particularly to a method for removing chlorine from a part of combustion gas extracted from a kiln exhaust gas passage, which runs from an inlet end of a cement kiln to a bottom cyclone, and efficiently removing lead from dust included in the extracted combustion gas.
2. Background Art
It is noticed that chlorine, sulfur, alkali and the like cause troubles such as preheater clogging in cement manufacturing facilities, and especially chlorine has the most harmful effect, so that cement kiln chlorine bypass systems are used in order to remove chlorine by extracting a part of combustion gas from a kiln exhaust gas passage, which runs from an inlet end of a cement kiln to a bottom cyclone.
In this chlorine bypass system, more chlorine is distributed on fine powder side of the dust, which is generated by cooling the extracted exhaust gas, so that the dust is separated into coarse powder and fine powder by a classifier, the coarse powder is returned to a cement kiln system, and the fine powder (chlorine bypass dust) containing separated potassium chloride and the like are recovered, and are added to a cement grinding mill system (as exemplarily described in the first patent document).
However, in recent years, recycling of waste through conversion to cement raw material or fuel has been promoted, which increases the quantity of harmful lead brought to cement kilns as the quantity of the treated waste increases. As a result, lead concentration in cement may exceed a management standard.
To remove heavy metal such as lead from chlorine bypass dust or the like, in the past, for example, a technique is described in the second patent document. The technique comprises the steps of: adding water to dust contained in kiln combustion gas; utilizing solid material deposited in the first slurry for a cement raw material and the like after adjusting the pH thereof to 12, which is optimum for depositing Cd in the dust; adding carbon dioxide to the first slurry to generate the second slurry of which pH is 9, which is optimum for depositing lead; and utilizing solid material deposited in the second slurry for a cement raw material and the like.
In addition, in order to provide a treating method for effectively recovering lead and the like included in waste material, a method is described in the third patent document. The method comprises the steps of: adding sulfuric acid, hydrochloric acid, nitric acid or carbon dioxide to filtrate, which is generated after washing waste material, including heavy metals so as to deposit the heavy metals included in the filtrate as hydroxide, sulfate or carbonate; eluting the heavy metals through alkali leaching of deposited sludge; and neutralizing or sulfurizing the filtrate to recover the heavy metals including lead in the filtrate through deposition.
Further, in order to effectively separate and remove chlorine-containing compounds and lead-containing compounds included in waste material, a method is described in the fourth patent document. The method comprises: a washing process for washing waste material; an alkaline elution process for alkaline-eluting filtered solid material; a lead removal process for dividing lead from the filtrate through deposition; a calcium removal process for dividing calcium from the filtrate from which lead is removed by depositing the calcium; and a chloride recovery process for recovering chloride by heating the filtrate to separate chloride.
In addition, in the fifth patent document, to improve heavy metal recovery efficiency while maintaining high desalting effect and controlling elution of heavy metals at the washing treatment of waste material such as dust, a method for treating waste material is described. The method controls elution of heavy-metal salt and desalts waste material while adjusting pH of water-soluble slurry (suspension with water-soluble solid) included in the waste material between 8.5 and 13 in a treatment process for desalting waste material including chlorine and heavy metals.
Patent document 1: World Patent Publication WO97/21638 pamphlet
Patent document 2: Japanese Patent 2764508 gazette
Patent document 3: Japanese Patent Publication 2002-11429 gazette
Patent document 4: Japanese Patent Publication 2003-1218 gazette
Patent document 5: Japanese Patent Publication 2002-18394 gazette